In drawing up those two masterly chapters of the "Origin", "On the Imperfection of the Geological Record," and "On the Geological Succession of Organic Beings", Darwin had need of all the experience and knowledge he had been gathering during thirty years, the first half of which had been almost wholly devoted to geological study. The most enlightened geologists of the day found much that was new, and still more that was startling from the manner of its presentation, in these wonderful essays. Of Darwin's own sense of the importance of the geological evidence in any presentation of his theory a striking proof will be found in a passage of the touching letter to his wife, enjoining the publication of his sketch of 1844. "In case of my sudden death," he wrote, "...the editor must be a geologist as well as a naturalist." ("L.L." II. pages 16, 17.)In spite of the numerous and valuable palaeontological discoveries made since the publication of "The Origin of Species", the importance of the first of these two geological chapters is as great as ever. It still remains true that "Those who believe that the geological record is in any degree perfect, will at once reject the theory"--as indeed they must reject any theory of evolution. The striking passage with which Darwin concludes this chapter--in which he compares the record of the rocks to the much mutilated volumes of a human history--remains as apt an illustration as it did when first written.
And the second geological chapter, on the Succession of Organic Beings--though it has been strengthened in a thousand ways, by the discoveries concerning the pedigrees of the horse, the elephant and many other aberrant types, though new light has been thrown even on the origin of great groups like the mammals, and the gymnosperms, though not a few fresh links have been discovered in the chains of evidence, concerning the order of appearance of new forms of life--we would not wish to have re-written.
Only the same line of argument could be adopted, though with innumerable fresh illustrations. Those who reject the reasonings of this chapter, neither would they be persuaded if a long and complete succession of "ancestral forms" could rise from the dead and pass in procession before them.
Among the geological discussions, which so frequently occupied Darwin's attention during the later years of his life, there was one concerning which his attitude seemed somewhat remarkable--I allude to his views on "the permanence of Continents and Ocean-basins." In a letter to Mr Mellard Reade, written at the end of 1880, he wrote: "On the whole, I lean to the side that the continents have since Cambrian times occupied approximately their present positions. But, as I have said, the question seems a difficult one, and the more it is discussed the better." ("M.L." II. page 147.) Since this was written, the important contribution to the subject by the late Dr W.T. Blanford (himself, like Darwin, a naturalist and geologist) has appeared in an address to the Geological Society in 1890;and many discoveries, like that of Dr Woolnough in Fiji, have led to considerable qualifications of the generalisation that all the islands in the great ocean are wholly of volcanic or coral origin.
I remember once expressing surprise to Darwin that, after the views which he had originated concerning the existence of areas of elevation and others of subsidence in the Pacific Ocean, and in face of the admitted difficulty of accounting for the distribution of certain terrestrial animals and plants, if the land and sea areas had been permanent in position, he still maintained that theory. Looking at me with a whimsical smile, he said: "Ihave seen many of my old friends make fools of themselves, by putting forward new theoretical views or revising old ones, AFTER THEY WERE SIXTYYEARS OF AGE; so, long ago, I determined that on reaching that age I would write nothing more of a speculative character."Though Darwin's letters and conversations on geology during these later years were the chief manifestations of the interest he preserved in his "old love," as he continued to call it, yet in the sunset of that active life a gleam of the old enthusiasm for geology broke forth once more.
There can be no doubt that Darwin's inability to occupy himself with field-work proved an insuperable difficulty to any attempt on his part to resume active geological research. But, as is shown by the series of charming volumes on plant-life, Darwin had found compensation in ****** patient and persevering experiment take the place of enterprising and exact observation; and there was one direction in which he could indulge the "old love" by employment of the new faculty.
We have seen that the earliest memoir written by Darwin, which was published in full, was a paper "On the Formation of Mould" which was read at the Geological Society on November 1st, 1837, but did not appear in the "Transactions" of the Society till 1840, where it occupied four and a half quarto pages, including some supplementary matter, obtained later, and a woodcut. This little paper was confined to observations made in his uncle's fields in Staffordshire, where burnt clay, cinders, and sand were found to be buried under a layer of black earth, evidently brought from below by earthworms, and to a recital of similar facts from Scotland obtained through the agency of Lyell. The subsequent history of Darwin's work on this question affords a striking example of the tenacity of purpose with which he continued his enquiries on any subject that interested him.